In recent years, higher resolution is being demanded of a reduced projection aligner (stepper) for use in photolithograpic semiconductor manufacturing in order to increase integration of semiconductor devices. As a means to increase the photolithograpic resolution of the above stepper, it has been suggested that the wavelength of the light source used in the stepper should be shortened. Currently, a stepper oscillates a light beam using a high-power excima laser as the light source in a shorter wave spectrum compared to that of a mercury lamp. The optical system of this type of stepper is formed of a combination of various optical devices such as a lens and mirror.
The mirror is used for returning and bending the light beam in the optical system of the stepper. A convex or concave mirror can be formed having the necessary imaging capacity provided the mirror has sufficient refractive index in relation to a range of incident angles of the light beam for either a convergent beam or a divergent beam.
Two known examples of a conventional mirror for an ultraviolet laser are shown in FIGS. 5 and 7 respectively.
A first example of a conventional mirror is shown in FIG. 5 depicting a structure including a dielectric multi-layer film 13 positioned on a substrate 11. The dielectric multi-layer film 13 comprises alternating layers of high refractive index film and low refractive index film. The optical thickness of each layer in the dielectric multi-layer film 13 at a given incident angle .theta. is .lambda./4 determined by the following equation: ##EQU1##
In other words, the optical thickness at a given incident angle is periodic at .lambda./4 such that high reflectivity can be obtained at the given incident angle. Also, the period of about .lambda./4 is optimized by a calculation using a calculator.
The incident angle characteristics of a laser mirror of the first example at .lambda.=193.4 nm is shown in FIG. 6. The s-polarized beams (Rs) and p-polarized beams (Rp) of FIG. 6 that yield over 95% reflectivity (% reflectance) lie within a narrowly designated incident angle range. For this example the range of incident angle which will yield over 95% reflectivity is between 35.degree. and 52.degree.. At all other angles of incidence the percent reflectivity drastically decreases such that a mirror of this type is not suitable in cases where a high reflectivity yield is desired over a wider range of incident angle.
A second example of a conventional mirror is shown in FIG. 7 depicting a structure including an Al film 12 of about 2000 .ANG. formed on a substrate 11. A protective film 14 of e.g. MgF.sub.2 of about 1500 .ANG. is formed over the Al film 12 to prevent deterioration of the Al film 12 by oxidation.
The incident angle characteristics of a laser mirror of the second example at .lambda.=193.4 nm is shown in FIG. 8. As shown in FIG. 8 when the incident angle increases, the reflectivity of the s-polarized beam Rs increases, and the reflectivity of the p-polarized beam Rp decreases such that polarization components are separated. When such a laser mirror is used in the optical system of a stepper, imaging performance tends to fluctuate. Also, a mirror with low reflectivity yields causes the amount of light to decrease such that efficiency in exposure declines. In addition, lowered resistance of the laser and deformation of the mirror surface tend to be caused by absorption of heat.
The above problem can be more prominent as the number of mirrors increase.
The present invention solves the above problems using a mirror for an ultraviolet laser structured to provide excellent incident angle characteristics in the ultraviolet spectrum so as to be highly reflective to a laser beam in the ultraviolet spectrum over a wide range of incident angles and to provide little fluctuation in polarization reflectivity in relation to a change in the incident angle i.e., to possess small separation in its polarization component(s).